1. Field of the Invention
This invention relates to a display apparatus for a vehicle, and more particularly, to a display apparatus for a vehicle, and even more particularly to a vehicle head up display (hereinafter referred as a "HUD") with a hologram for reflecting display light from a displayer.
2. Description of the Prior Art
Conventionally, a HUD-type display apparatus for a vehicle with a hologram has been used in which the hologram having a reflecting function is installed, for example, in a windshield. The hologram reflects display light from a displayer toward a view point E of a driver, permitting a virtual image of the displayer to be displayed behind and far from the windshield. For instance, as illustrated in FIG. 4, a displaying surface 1a of the displayer 1 is installed in the dashboard 2 with the displaying surface facing the windshield 3. A hologram 4 having a reflecting function is attached to an inner wall of the windshield 3 so that display light emitted from the displaying surface 1a of the displayer 1 passes through an opening 2a of the dashboard 2 and reflects off the hologram toward the view point E of the driver. As a result, when observed by the driver, a virtual image X.sub.1 of the displayed image is recognized behind and far from the hologram 4.
As described above, using the hologram as a reflecting plate has the advantage of providing an image with high luminance, since light in a particular range of wavelengths is reflected efficiently. Secondly, the hologram may perform other optical functions, such as lens and concave mirror functions, which allows the viewing field to be widened or displaying system to be miniaturized. Thirdly, unlike an ordinary mirror or windshield, which can be used as a reflecting plate, using the hologram as the reflecting plate permits optical axes to be designed flexibly besides regular reflection with the same incident and reflection angles.
However, when sun light enters a hologram attached to the inner wall of a windshield as described above, flare or a ghost image is generated, which reduces the recognizability of the holograhpic image. Further, when observed from outside of the vehicle, the hologram appears to be colored, which is not preferable in appearance. Moreover, when the hologram is exposed to sun light for a long period of time, it may be deformed due to ultra violent rays, and infrared rays, and the like.